In the present era of environmental awareness, the gas turbine engine designer, and particularly the designer of such engines for aircraft propulsion, is faced with the dilemma of reducing engine pollutants with a minimum sacrifice of engine performance. One type of pollution which recently has received considerable attention is noise.
Gas turbine engine noise is generated from two primary sources: first, there is that associated with the viscous shearing of rapidly moving gases exhausted into the relatively quiescent surrounding atmosphere. In turbofan aircraft engines, such gases are emitted from the fan and core nozzles at the rear of the engine. Various approaches have been utilized to reduce this "shear" noise, most approaches incorporating mixers to comingle fan and exhaust gases with each other and with the surrounding environment.
The second source of noise, and the one to which the present invention is directed, is generated by the rotating turbomachinery itself, the result of rapidly rotating blade rows disposed within the gas stream. The noise is affected by such parameters as blade rotational speed, blade-to-blade spacing, blade geometry and also by the proximity of stationary hardware to such rotating blade rows, as in the case of an outlet guide vane arrangement and in typical multistage axial compressors where stationary blade rows are alternated with rotating blade rows. Some of the noise generated in this manner can be absorbed and suppressed by means of acoustic or sound absorbing paneling disposed about the periphery of the nacelle enclosing the rotating turbomachinery. Such sound-absorbing material is well known in the art. However, a significant percentage of noise propagates forward from the gas turbine inlet duct due to the proximity of the fan or compressor to the inlet frontal plane and the lack of forward shielding in the forward direction. The problem, therefore, facing the gas turbine designer is to provide a means for attenuating this forward propagating noise without incurring overall performance penalties.
Prior state of the art concepts to attempt to solve this problem have concentrated on the addition of sound-absorbing material upon the inlet duct inner wall. This does little to attenuate unreflected noise propagating in the axially forward direction. Additional benefits have been obtained by providing coaxial, circumferential rings of sound absorbent material within the inlet. However, such rings produce a loss of inlet total pressure and, therefore, performance losses which remain throughout the engine operating envelope even where noise propagation presents no hazard or nuisance to inhabitants below.
Another concept incorporates an axially translating scoop on the bottom of the inlet duct to selectively reduce the downward transmission of noise from the inlet. However, this concept does little to protect against the nuisance of noise in one of the most critical engine operating regimes; that is, when the engine is at ground altitude. Downward propagating noise in this environment is shielded by the ground itself, while side propagating noise remains unattenuated.